It is known that the efficiency or efficacy of a catalytic converter treating the exhaust gas of an engine is significantly effected by the ratio of air to fuel supplied to the engine. At stoichiometric ratio, catalytic conversion efficiency is high for both oxidation and reduction conversions. The air/fuel stoichiometric ratio is defined as the ratio of air to fuel which in perfect combustion would yield complete consumption of the fuel. The air/fuel ratio LAMBDA of an air/fuel mixture is the ratio of (a) the amount by weight of air divided by the amount by weight of fuel, to (b) the air/fuel stoichiometric ratio. Closed loop fuel control systems are known for use in keeping the air/fuel ratio in a narrow range about the stoichiometric ratio, known as a conversion window.
It is known that the efficiency of catalytic conversion is affected by the oxygen storage capability of the catalytic converter. A properly operating catalytic converter dampens oxygen concentration fluctuations in the exhaust stream. A system proposed in SAE paper No. 900062, Detection of Catalyst Performance Using On-Board Diagnostics, employs two exhaust gas oxygen sensors, one upstream and one downstream of the catalytic converter, to detect oxygen content in the exhaust gas The system employs test signals in the form of an air/fuel ratio swing on both sides of stoichiometry at predetermined rates or frequencies caused by fuel control system perturbations. By comparing the change in response patterns between the upstream and downstream EGO sensors, a determination can be made about catalytic converter efficacy. In such system the monitoring results depend on signals from two EGO sensors which may have different characteristics, due either to manufacturing tolerances or to differential aging over a period of use, leading to possible errors in the results.
Accordingly, it is desirable to have a catalytic converter monitoring system and method which are reliable and do not depend on signals from both upstream and downstream EGO sensors. It is particularly desirable to incorporate such monitoring system and method in a fuel control system utilizing both upstream and downstream EGO sensors.